Polyesters are now widely used in the manufacture of fibers for textiles and other applications. One such polyester, polyethylene terephthalate (PET), is produced world-wide in billions of pounds per year. PET is typically made by the condensation of terephthalic acid (TA) or dimethylterephthalate (DMT) with ethylene glycol. While PET has many desirable properties that make it suitable for manufacturing fibers, there is a continuing need for new polyester fibers that have improved properties, or properties that are different from PET, thereby opening new uses for polyester fibers. Since PET is manufactured worldwide in such large amounts for application in textiles as well as in, for example, packaging for liquids, there is also a need to find uses for recycled PET.
Although 2,6-naphthalenedicarboxylic acid (NDA) and its dimethyl ester, dimethyl-2,6-naphthalenedicarboxylate (NDC), have been known for many years, there has been great commercial interest recently in polyesters made from NDA or NDC. For example, polyethylenenaphthalate (PEN) prepared by condensing NDA or NDC with ethylene glycol is a high performance polyester that is useful in film and fiber applications. As film, it has superior gas barrier properties and as a fiber it has superior tenacity. NDC is now commercially available from Amoco Chemical Company, and it is currently being used in the commercial manufacture of bottles and films. Thus, in addition to recycling PET, there is also a need to find uses for recycled PEN or other polyesters containing the 2,6-naphthalate unit.
The present invention, which is a polyester fiber having both terephthalate and 2,6-naphthalate units, is an improved fiber in that it has high shrink properties which makes it useful in fiber applications where crimp retention or high bulk is desired, such as in carpet yarns, "hi-loft" non-woven fabrics used as interlinings, cushioning and filtration media, as well as in specialty yarns for weaving and knitting. The fibers of this invention also have a lower melting temperature compared to PET which makes them useful as binder fibers in non-wovens, particularly in combination with PET homopolymer fibers. Since the polyester fibers of this invention can be prepared from blends of polymers, for example, a blend of PET with PEN, or a blend of PET with a copolymer having terephthalate and naphthalate units, the fibers of this invention can be made from recycled PET and PEN, or by blending recycled PET with copolyester containing terephthalate and 2,6-naphthalate units, thus providing a valuable use for recycled polyester materials.
Fibers made from PET modified with isophthalate units have been described in Amoco Chemical Company Bulletin GTSR-113A, November 1995, "PET Modified with Purified lsophthalic Acid for Shrink Fiber Applications." Relative to such isophthalic acid modified PET, the fibers of the present invention have a higher glass transition temperature (Tg) making them more suitable for certain fiber applications such as filters for filtration of hot gases. Additionally, the incorporation of naphthalate units in PET provides for a polyester with improved thermal, oxidative and hydrolytic stability.